The jet mill jets compressed air or superheated steam of a pressure several times the atmospheric pressure, or a high-pressure gas in jet streams through a jet nozzle to accelerate particles to pulverize the particles by collision or abrasion between the accelerated particles or by collision of the accelerated particles against a collision plate.
The jet mill is a dry pulverizer capable of pulverizing particles into finely pulverized product having a mean particle size of several micrometers and is suitable for pulverizing heat-susceptible substances.
There are some types of jet mills including swirling-current jet mills, opposed nozzle jet mills, fluidized bed jet mills and impactor jet mills. Those jet mills, irrespective of type, are provided with a particle feed apparatus for feeding particles into a fluid jet.
FIG. 11 is a view of assistance in explaining the operation of a conventional horizontal swirling-current jet mill provided with ancillary devices for pulverizing particles.
Referring to FIG. 11, a jet mill 01 has a horizontal swirl casing. Compressed air is jetted through a plurality of pulverizing nozzles 02 into the swirl casing to produce a swirling current in the swirl casing. A ring header pipe 03 holding compressed air is connected to the plurality of pulverizing nozzles 02 to supply compressed air to the pulverizing nozzles 02. A compressor 05 is connected through a pressure regulator 06 and a valve 07 to the ring header pipe 03 to supply compressed air to the ring header pipe 03.
An air injector 04 discharges a solid-gas mixed fluid containing carrying air and particles into the horizontal swirling casing of the jet mill 01.
The compressor 05 is connected through a pressure regulator 010 and a valve 011 to the air injector 04 to supply compressed air to the air injector 04. The compressed air supplied to the air injector 04 is accelerated and jetted as carrying air through a venturi nozzle, not shown, into the jet mill.
The air injector 04 is provided with a feed hopper 04a. A feeder 015 feeds particles through a connecting chute 016 at a regulated feed rate into the feed hopper 04a. 
The feeder 015 has a particle hopper 015a opening into the atmosphere, and the connecting chute 016 has a sidewall provided with an opening 016a. Thus, the feeder 015 and the connecting chute 016 form an open particle feed apparatus.
In the jet mill 01 combined with those ancillary devices, the venturi nozzle of the air injector 04 accelerates and discharges carrying air into the swirl casing of the jet mill 01 to produce a partial vacuum on its upstream side. Consequently, air around the jet mill 01 is sucked as secondary air through the opening 016a of the connecting chute 016 together with the particles fed at a fixed feed rate by the feeder 015. The secondary air and the particles are mixed with carrying air to produce a solid-gas mixture, and the solid-gas mixed fluid is accelerated and introduced into the jet mill 01.
The particles accelerated by the venturi nozzle of the air injector 04 and discharged into the swirling casing are pulverized by collision and abrasion between the particles and collision against, and abrasion by the wall of the swirl casing.
The collision speed of the particles carried by the carrying air and colliding against each other must be high to enhance the pulverizing ability of the swirl jet mill. Generally, the horizontal swirl jet mill is designed and fabricated so as to produce a high-speed swirling-current.
The static pressure of the swirling current swirling at a high velocity in the jet mill is high. If the static pressure of the swirling current exceeds the vacuum (negative pressure) produced by the air injector, the air injector is unable to blow particles into the swirling current and backflow occurs.
Therefore the jet mill 01 must be operated by the following procedure to avoid backflow.
First, the valve 011 connected to the air injector 04 is fully opened and compressed air is supplied to the air injector 04.
Then, the valve 07 placed in the line connected to the pulverizing nozzles 02 is opened gradually to increase the pressure gradually from zero (gage pressure). The opening operation for opening the valve 07 is stopped when the pressure of compressed air at the pulverizing nozzles 02 has reached a value that produces a swirling current of a static pressure not exceeding the vacuum in the air injector 04 in the jet mill 01.
If the static pressure of the swirling current exceeds the vacuum in the air injector 04 and backflow occurs in the feed hopper 04a of the air injector 04, the opening of the valve 07 is reduced slightly to reduce the pressure of compressed air at the pulverizing nozzles 02 to a level that does not cause backflow.
In this state, a swirling current is produced in the jet mill according to the pressure at the pulverizing nozzle 02. Then, secondary air sucked through the opening 016a of the connecting chute 016 is blown together with the carrying air through the venturi nozzle into the jet mill.
Then, the feeder 015 starts feeding particles at a fixed feed rate. The particles and the secondary air are mixed with the carrying air to produce a solid-gas mixed fluid, and the solid-gas mixed fluid is blown into the swirling current produced in the jet mill.
The particles introduced into the swirling current collide against each other and are pulverized. When the particles are thus introduced into the swirling current, the velocity of the thus loaded swirling current decreases and the static pressure of the swirling current drops below the static pressure of the unloaded swirling current.
It takes time in the range of several minutes to ten-odd minutes until a stable swirling current having a fixed particle holdup is produced in the jet mill while particles are fed at the fixed feed rate. The existing state is maintained for that time.
After a stable swirling current has been produced, the valve 07 connected to the pulverizing nozzles 02 is opened gradually to increase the pressure at the pulverizing nozzles gradually so that the velocity of the swirling current increases. Consequently, the milling effect of the swirling current increases for the finer pulverization of the particles.
The static pressure of the swirling current increases in proportion to the velocity of the swirling current. Increase of the opening of the valve 07 is stopped immediately before the static pressure of the swirling current of the solid-gas mixed fluid containing particles exceeds the vacuum of the air injector 04 and the secondary air is sucked at a low rate through the opening 016a of the connecting chute 016 while the jet mill is in operation.
The backflow of powder-laded air occurs when the static pressure of the swirling current of the solid-gas mixed fluid exceeds the vacuum of the air injector 04. Therefore, the operator empirically determines the time to stop the operation for opening the valve 07 immediately before the static pressure exceeds the vacuum.
Thus, the valves need to be operated by many sequential steps to prevent the backflow of powder-loaded air due to the increase of the static pressure of the swirling current beyond the vacuum produced by the air injector, and hence the jet mill is very difficult to operate.
Although it is possible to measure the static pressure at which backflow occurs of the swirling current of air not loaded with particles, the static pressure of the swirling current of the solid-gas mixed fluid cannot be measured while the particles are fed at an optional feed rate. Therefore, the difference between the static pressure of the swirling current immediately after the start of feeding the particles and the static pressure at which backflow occurs cannot previously be determined. The operator needs to visually watch the opening 016a of the connecting chute 016 while the operator is operating the valve 07 for the finer pulverization of the particles and needs to reduce the opening of the valve 07 slightly upon the detection of any indication of backflow to prevent backflow and to set the valve at that opening. Thus, the conventional jet mill is operated by such an instable operating method depending on the visual detection of the operating condition and the operator's experience
The static pressure of the swirling current varies in a wide range in a transient state between the start of operation of the jet mill and the achievement of the predetermined particle holdup in the swirling current. Therefore, the opening of the valve 07 needs to be adjusted repeatedly so that the highest possible static pressure of the swirling current that does not cause backflow is maintained. Thus, the operation of the valve 07 is troublesome.
Even if the static pressure of the swirling current is stabilized at a level slightly below the static pressure at which backflow occurs after the achievement of the particle holdup, the load on the swirling current decreases if the feed of particles by the feeder 015 is interrupted or the feed rate is reduced by some cause. Consequently, the velocity of the swirling current rises, the static pressure of the swirling current rises accordingly, and, in some cases, the static pressure increases beyond the vacuum in the air injector 04 causing backflow.
If backflow occurs, particles and powder are blown out and through the opening 016a of the connecting chute 016 and are scattered in and contaminate the workshop, which is detrimental to the health management of the operators. Thus, backflow is a serious problem to be solved.
Since the operation of the jet mill must be stopped and materials are wasted if backflow occurs, the pulverizing process must continuously or frequently be monitored to see if there is any indications of backflow. Such a pulverizing process is inefficient and practically disadvantageous.
A pulverizing method may feed particles at a low feed rate to control the particle size in pulverizing particles by a jet mill. When the feed rate is reduced, load on the swirling current decreases. Consequently, the velocity of the swirling current increases, the static pressure of the swirling current increases accordingly and, eventually, backflow is liable to occur.
Backflow can be prevented only by controlling the pressure of the compressed air at the pulverizing nozzles 02 so that the static pressure of the swirling current may not exceed the vacuum in the air injector 04.
It is one of original pulverizing methods using a jet mill that maintains a high-velocity swirling current and feeds particles into the swirling current at a low feed rate so that the particle concentration of the swirling current is small and load on the jet mill is small.
However, the pressure of compressed air at the pulverizing nozzles inevitably needs to be reduced because backflow is liable to occur if the velocity of the swirling current is high. Thus, the conventional pulverizing method is unable to achieve pulverization satisfactorily under pulverizing conditions requiring feeding particles at a low feed rate and producing a high-velocity swirling current.
The present invention has been made in view of those problems and it is therefore an object of the present invention to provide a particle feed apparatus for feeding particles to a jet mill, capable of preventing backflow, improving the condition of the working environment, and enabling the jet mill to fully exercise its pulverizing ability, and facilitating the operation of the jet mill.